If you remove the cover from the fly wheel you should be able to see a turn buckle. turn the buckle so it brings the magnet close to the fly wheel that gives it more resistance.
you will need to loosen the nuts on the turn buckle to adjust it.
Having had home exercise equipment, you'll probably just damage the machine. I do find it odd that it would have so little resistance -- maybe you're not using it properly? But for best results, either go to the gym where they have better bikes or buy a heavier duty one. Or, and I know this is radical these days, buy a real bike, go OUTSIDE, and ride hills.